1. Field of the Invention
The invention relates to modular expansion boards for leveraging a single circuit across multiple expansion boards and more particularly to expansion boards of alternate orientations.
2. Background of the Invention
Since the first personal computer (PC) appeared on the market, the ability of the PC to expand its functionality has been a significant factor to its success. From the beginning, each PC included an expansion bus for adding functionality. Many familiar functions, such as graphics, network communications and telecommunications have typically been found on expansion boards. This allows the computer user to purchase a relatively low priced computer and later upgrade the system with certain functionality available on an expansion board.
For many years, the primary expansion bus has been the ISA bus (for Industry Standard Architecture). The ISA bus has a 24-bit address bus and an 8 or 16 bit data bus and operates at a maximum frequency of 8.33 MHz. The system board of the PC includes connectors, or slots, for receiving up to eight ISA expansion boards. An ISA expansion board has a edge connector on its bottom edge for connecting to the system board connector. The system board connectors are oriented so that the ISA expansion board extends up perpendicularly from the system board when mounted, with one edge being adjacent the rear of the PC. An opening at the rear of the PC adjacent each expansion board allows cables to be attached to the ISA expansion board. Each system board connector is mounted adjacent another and each ISA expansion board is allotted a certain width which restricts components except for simple surface mount resistors and capacitors, to one side of the expansion board. Thus, this arrangement permits a series of ISA expansion boards to be connected in a small amount of space.
As PCs became more networked and reliant on file servers, the ISA bus became a bottleneck. Additionally, as 32-bit processors became the mainstream processor, the 16-bit ISA bus was deemed inadequate. Thus, an improvement to the ISA bus, known as the microchannel architecture bus (MCA), was developed by International Business Machines (IBM). The MCA bus provides full 32-bit data, and an improved data transfer mechanism. However, the MCA bus diverged significantly from the ISA bus, and therefore does not maintain backwards compatibility with the ISA bus. The MCA bus utilizes a different connector from that of the ISA bus.
A short time after the MCA bus was developed, another improved ISA bus was developed to compete with the MCA bus, known as the EISA bus (for Extended ISA). The EISA bus provides backwards compatibility with the ISA bus and extends the 16-bit data bus to 32-bits, the 24-bit address bus to 32-bits, and improves on the Direct Memory Access transfers (DMA). Physical compatibility is maintained by placing the additional EISA pins between the predefined ISA pins. Thus, an ISA expansion board can be inserted into an EISA board slot.
As expansion board manufacturers now had to support as many as three different expansion buses, they sought ways to manufacture a single board for more than one bus. One known attempt at this was performed with expansion boards designed to communicate with IBM mainframes, as described in U.S. patent application Ser. No. 5,163,833, entitled "Dual Personal Computer Architecture Peripheral Adapter Board". These dual bus expansion boards, contain a single circuit for communication with mainframe computers. The expansion board contains an MCA connector along one edge, an ISA connector along the opposite edge and a connector for receiving a removable board. The removable board is placed into the expansion board in one of two positions, depending on the type of expansion bus utilized. The components are mounted on a side consistent with MCA expansion board requirements, however, when the dual bus board is flipped over for use as an ISA board, the components are inconsistent with ISA expansion board requirements. It is also worth noting that in the dual bus boards, since only one expansion bus connector is utilized at a time, one connector is always left exposed. Thus, the possibility for electrical shorting and accelerated corrosion exists for the exposed contacts. A illustration of the general shape of a dual bus board is shown in prior art FIG. 1A. FIG. 1C illustrates the dual bus board in a computer having one of the bus architectures.
A known method exists for utilizing one circuit board with multiple other circuit boards. In this method, a daughterboard is mounted to a main board. In one case, the daughterboard contains a common circuit for connecting to multiple different main boards. In another case, the daughterboard contains different circuits for connecting to one or multiple main boards having a common daughterboard interface. Daughterboards are typically smaller circuit boards for mounting directly onto another larger main circuit board. The daughterboard typically has a pair of headers on a side opposite its component side, with one header mounted at each end of the daughterboard. The main board has a pair of female-ended connectors for receiving the daughterboard. When connected, the bottom surface of the daughterboard lies very close to the top surface of the main board, but not in the same plane. This allows a single main board to carry different variations of daughterboards, or a single daughterboard to connect to different variations of main boards. This type of arrangement is known to exist on expansion boards and system boards. However, a daughterboard for an expansion board has limited use since the width of the expansion board is limited. Thus, the height of components for a daughterboard is reduced by the distance from the surface of the main board to the surface of the daughterboard. So many components cannot be utilized with this arrangement.
As PCs became more graphically oriented, it became desirable to place some of functionality typically found on the expansion buses directly on the processor bus to achieve maximum performance. Since the processor provides limited buffer drive capability, this method has many limitations. This eventually led to another bus proposal known as the Peripheral Component Interconnect (PCI) bus. The PCI bus provides a level of performance and user convenience not available from the previous buses. Backwards compatibility was not an objective, thus another bus interface was developed. Expansion boards for the PCI bus are similar in size to ISA and EISA expansion boards. However, the connector is different from previous bus connectors and component placement is on the opposite side from the ISA/EISA expansion boards.
This again presented a problem for manufacturers that desired to create an expansion board for the PCI bus and at least one other. The methodology utilized with the prior dual bus boards was not satisfactory because the dual bus boards actually exceeded the slot spacing requirements and were more costly since they have an unused connector. The daughterboard/mainboard configuration had height limitations. Therefore a solution is necessary which allows the flexibility of daughterboards, but without the inherent height limitation, and usable where components appear on differing sides of the circuit boards for different buses.